9. article azojete vol 11 103 113 dibal

Arid Zone Journal of Engineering, Technology and Environment. August, 2015; Vol. 11: 103-113
Copyright© Faculty of Engineering, University of Maiduguri, Nigeria.
Print ISSN: 1596-2490, Electronic ISSN: 2545-5818
www.azojete.com.ng
DESIGN OF A LOCK-IN AMPLIFIER MICRO-OHMMETER USING PROTEUS VSM
Dibal, P. Y*. and Mshelia, D. E.
(Department of Computer Engineering, University of Maiduguri, Nigeria)
*Corresponding author’s email address: yoksa77@gmail.com
Abstract
This paper presents the design of a lock-in amplifier micro-ohmmeter using Proteus VSM. To achieve a seamless
complex design, it is usually better and easier to break the design task into stages and this paper used that approach
in Proteus to achieve the task of designing the micro-ohmmeter. The paper introduced the concept of a micro-
ohmmeter and its constituent segments. Each segment was analyzed at component level with appropriate
mathematical equations. The result and implementation of each analysis was verified through simulation in Proteus.
The paper makes a comparison between the theoretical operation of the circuit and the operational result obtained
via the simulation. The comparison confirms that the design of the circuit using Proteus was successful due to the
fact that the simulation results matched that of theory.
Keywords: Micro-ohmmeter, Proteus, simulation, circuit design
1. Introduction
The increase in the demand for very accurate measuring instruments is motivating the design of
highly sensitive instruments like the micro-ohmmeter with the ability to measure very small
resistances in the presence of large noise. The more highly accurate measuring instruments there
are, the more highly precise electronic components and devices can be designed and produced,
thus enhancing reliability of such components. In sophisticated and advanced applications of
electronics, integrity of signals and installation cannot be taken for granted once they have been
ascertained. The integrity of component parts of such installations can be reliably measured
using devices like the micro-ohmmeter.
The purpose of this paper is to design and test a lock-in amplifier based micro-ohmmeter, which
has the capability of measuring very small resistances without the application of large currents.
To achieve this, the device is specified to measure a maximum resistance of 0.22Ω ±20% with a
supply voltage of 6V. The excitation oscillator is specified to output a frequency of 275Hz ±10%
and an unloaded output signal of ETH = 200mV (Bateson, 2011).
1.1 Overview of Instrument
The lock-in amplifier based micro-ohmmeter is made of four stages, starting from the source to
the output:
 The source stage is made of an excitation oscillator, a quadrature divider and an
attenuator.
 The instrumentation amplifier stage which has a buffered amplifier, a phase reversing
switch, and a differential amplifier.
 The filter stage which has a Sallen-key equal value Butterworth lowpass filter to remove
noise in the form of mains pick-up.

Dibal and Mshelia: Design of a Lock-in Amplifier Micro-Ohmmeter using Proteus VSM. AZOJETE, 11:
103-113
104
 The output stage which has a Voltage Controlled Oscillator (VCO), whose oscillation is
muted by a transistor in the off-state. This is achieved using a muting detector circuit.
(Bateson, 2011).
The block diagram of the interconnectivity between the different stages is shown in Figure 1.
2. Sub-Circuits Design and Analysis
In this section, the detailed design and analysis of each of the four stages of the micro-ohmmeter
is carried out.
2.1 Source Stage
This stage is made of a 555 timer, 4013 frequency divider, and an attenuator. The specifications
for this section are:
Vin = 6V, Vout = 200mV ±5%, Input frequency (fin) = 1.1kHz, Output frequency (fout) = 275Hz.
2.2 The 555 Timer
The 555 timer (Horowitz and Winfied, 1989) is connected as an oscillator in a stable mode in
this stage. Figure 2 shows this connection:
Figure 2: The 555 timer in a stable mode
Figure 1: Block diagram showing interconnecting stages of the micro-ohmmeter

105
The capacitor C (Horowitz and Winfield, 1989) is discharged when power is applied, causing a
triggering of the 555 timer, and the output goes HIGH. The capacitor begins to charge towards
Vcc through R1 + R2. When it reaches 2/3 Vcc or Vs as shown in Figure 3, the THRESHOLD
input becomes triggered, causing the output to go LOW. The capacitor C is then discharged
towards ground through R2. The operation is now cyclic and the voltage of C goes between 1/3
Vcc and 2/3 Vcc, with a period of
T = 0.693(R1 + 2R2) C (1)
Figure 3: Charging and discharging of capacitor C
2.3 Quadrature Divider (4013 Frequency Divider)
The 4013 frequency divider used in this stage is in pairs, and both of them are triggered by the
555 timer. Figure 4 shows the 4013 divider and its output waveform. The function of the 4013
divider is to divide frequency of the signal at it clock input.
Figure 4: The 4013 frequency divider and output waveform
From the waveform of the frequency divider, by “feeding back” the output from Q’ to the input
terminal D, the output pulses at Q have a frequency f/2 that of the input clock frequency
(Horowitz and Winfield, 1989).
2.4 Attenuator
This is a resistive divider network used for precise control of amplitude. In this stage, the
amplitude being controlled is that of the signal from the quadrature divider. The attenuator is
shown in Figure 5.

103-113
106
Figure 5: Attenuator
The period T is given in Equation (1) as:
(1)
Where: f is the frequency = 1.1 kHz, from the given specification.
Therefore, . Substituting for T in equation (1),
. Setting R1 at 1kΩ
By the rule of voltage divider (Horowitz and Winfield, 1989):
Setting R3, and R4 to 1.1kΩ, Vout = 200mV from specification, and Vin = 6V from specification.
(2)
From the foregoing calculations, the design of the source stage is shown in Figure 6.

107
Figure 6: Design of the source stage for the micro-ohmmeter
When a 1mΩ resistor is connected across R5, the effective resistance becomes 1mΩ. By voltage
divider in equation (2), the output becomes 2.72µV. When a 0.22Ω resistor is connected across
R5, the effective resistance becomes 0.219Ω, and by equation (2), the output voltage is
calculated as 597µV.
However, due to the inherent forward voltage drop ((Horowitz and Winfield, 1989) from the
4013 frequency divider, the recalculated value of R5 becomes:
Therefore, R5 = 95.65Ω, and the nearest value of R5 is 91Ω.
This implies
2.5 Instrumentation Amplifier Stage
Instrumentation amplifiers have high gain differential input with high input impedance and they
have a single-ended output. The input stage is a configuration of op-amps that provides high
differential output representing a signal with substantial reduction in the comparative common-
mode signal, and is used to drive a conventional differential circuit (Horowitz and Winfield,
1989). Figure 7 shows the buffered stage of the instrumentation amplifier.

103-113
108
Figure 7: Instrumentation Amplifier
The relationship between the gain and the value of R9 from is given by:
(3)
From equation (3), we can see that the gain is distributed. This gives the advantage of an overall
increase in bandwidth. However, the disadvantage of such an action is that the gain will be at the
expense of Common Mode Rejection Ratio (CMRR) (Stephen, 2001)
Given the peak voltage amplitude of 1.2V at the output of U5A, and by equation (3), R9 is
calculated thus:
Where v1 = 477µV, and v2 = -477µV.
There R9 = 79.81Ω, and we choose the nearest value of 82Ω.
The 4053 phase reversing switch shown in Figure 7, has a single pole change-over switches,
which are simultaneously operated pairs of transmission gates. Since their switch actions are
simultaneously in opposite directions, they cancel most of the switch charge injection (Lee and
Philip, 2003).
2.6 Filter Stage
This is the stage in which we remove unwanted noise (switching noise, noise from main pickup,
thermal noise, and DC offset) coming from the instrumentation amplifier stage. It has the Sallen-
key equal value filter, which is a low-pass filter. Figure 8 shows the Sallen-key equal value filter.

109
Figure 8: Sallen key equal value filter
From (Bateson, 2011), Equation (4) shows that:
(4)
where: f0 is chosen to be 5Hz.
The nearest value for R18 = R20 = 330Ω.
2.7 Output Stage
This stage is the output of the system. It consists of a Voltage Controlled Oscillator (VCO).
2.7.1 Voltage Controlled Oscillator
The Voltage Controlled Oscillator (VCO) is an oscillator circuit whose frequency can be
controlled or varied by a DC input voltage. In this design, the VCO has an integrator whose
output is fed to a Schmitt trigger. The design of the VCO is realized in Figure 9 below:

103-113
110
Figure 9: Voltage Controlled Oscillator
When the output of the Scmitt trigger U6:B is in the low state i.e. approximately 0V (Figure
10a), it causes Q1 to be off (Figure 10b) and all the current supplied by R23 flows into capacitor
C9, thereby causing a down ramp of the output of U6:A. When this output from U6:A reaches
0V, a snapping of the Schmitt trigger occurs, which causes the output of U6:B to jump to
maximum voltage (Figure 11a). This turns Q1 ON (Figure 11b), shorting R26 to ground and
sinking a current , where Vvco is the input voltage to the VCO stage. But this current is twice
that suppled by R23; so the other half comes from C9. Because Q1 is ON and current flowing
through C9 is reversed, the output of U6:A is now ramped upward.
The frequency of the VCO is given as Equation (5):
(5)
Where VTH = upper threshold, VTL = lower threshold, R = R26 = 100kΩ.
Hence,

111
Figure 10a: U6:B in LOW state Figure 10b: Q1 turned OFF with base ≈ 0
Figure 11a: U6:B in HIGH state Figure 11b: Q1 turned ON
3. Simulation and Results
The simulation of the entire design (done with Proteus VSM) reveals the following results: the
output of the 555 timer shown in Figure 12 reveals the relationship between output of the timer
(pin 3) and the charge and discharge behavior of the capacitor (pin 6). The result obtained in
Figure 12 confirms that the design produced the expected result as shown in Figure 3.
Figure 12: Output of pin 3 and 6 of the 555 timer
The simulation result of the 4013 frequency divider shown in Figure 13 shows the output is half
that of the 555 timer Figure 12. The result obtained confirms that the design produced the
expected output depicted in Figure 4.

103-113
112
Figure 13: Output of 4013 frequency divider
The simulation of the output stage of the design is shown in Figure 14. The buzzer is used
because the design is conceived to give an audible sound as it makes small measurements. Figure
15 shows the complete system design.
Figure 14: Buzzer output
Figure 15: Complete System Design

113
4. Conclusion
In this design, the operating principle and simulation results of different segments of a micro-
ohmmeter were presented. The function of each segment and the effect they have on the signal
were discussed. Calculation of values for each component used in the design was performed and
presented in each subsection that deals with the analysis of each component. It should be noted
that the values obtained and used in design are ideal values; hence the output of the system will
be ideal. In practice however, there will be a variation in the output of the design because the
components selected and used will have slight tolerances. Using Proteus VSM, the design of the
Lock-in Amplifier Micro-Ohmmeter was successfully carried out and the result obtained in the
design matched the expected theoretical results.
References
Bateson, S. 2011. Electronic Signal Conditioning Laboratory Manual. eCircuit, 2011. Sallen-Key
Low-Pass Filter available at: http://www.ecircuitcenter.com/Circuits/opsalkey1/opsalkey1.html
[Accessed: 22nd
January, 2016]
Horowitz, P. and Winfield, H. 1989. The Art of Electronics. 2nd
edn. Cambridge: Cambridge
University Press.
Lee, H. and Philip, KT. 2003. Active-Feedback Frequency-Compensation Technique for Low-
Power Multistage Amplifiers. IEEE Journal of Solid State Circuits, 38(3): 511 – 520.
Sergio, F. 2002. Design with Operational Amplifiers and Analog Integrated Circuits. 3rd
edn.
New York: Mc-Graw Hill
Stephen, A. 2001. Survey of Instrumentation and Measurement. New York: John Willey & Sons.

9. article azojete vol 11 103 113 dibal

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (12)

Similaire à 9. article azojete vol 11 103 113 dibal

Similaire à 9. article azojete vol 11 103 113 dibal (20)

Plus de Oyeniyi Samuel

Plus de Oyeniyi Samuel (20)

Dernier

Dernier (20)

9. article azojete vol 11 103 113 dibal